This invention relates to a method and apparatus for spectrophotometric chemical analysis of a material contained within a moving stationary process stream. The invention described in this application is intended specifically to be used on-line in a manufacturing or other process environment where rapid analysis of the chemical composition of a material in a moving process stream is critical to efficient and economical quality control. The invention desclosed in this application has a very wide range of use. For purposes of description and illustration, the spectrophotometric apparatus uses infrared form of radiation in order to obtain an infrared spectrum for analysis according to the Fourier Transform Infrared Spectroscopy method. In this application this is referred to as "FTIR."
While chemical analysis of a wide range of materials is possible, the invention will be described for purposes of illustration with relation to the chemical analysis of a polymer melt contained and moving in a process stream such as in a polymer manufacturing facility or in a synthetic fiber manufacturing facility.
The infrared frequency range (2.5 to 50 microns or 4,800 to 200 wave numbers) has been used in infrared spectroscopy for some time. The popularity of FTIR infrared radiation analysis as an analytical tool is the result of the relatively large amount of information that infrared spectroscopy provides and the manner in which it can be generated and analyzed. It is most widely used for the identification of all organic compounds and many non-organic compounds, and is useful because it can analyze a sample whether in the solid, liquid molten or gas phase and whether the materials are pure or impure. Depending on the manner of use, both qualitative and quantitative information can be provided. Analysis of infrared radiation output data is relatively rapid, lending itself to at least theoretical use in on-line processes. However, the physical problems associated with testing of materials in an on-line environment have been difficult. Accordingly, most infrared analysis has been and still is conducted in a laboratory environment. In the polymer manufacturing and processing environment described above, the typical method of qualitative analysis of the polymer melt is to take a sample from the end product, i.e., flake or pellet and deliver it to a laboratory location for infrared analysis. This procedure is particularly unsuitable because of the nature of the polymer manufacturing process. Polymers, such as polypropylene, polyethylene, nylon and the like are produced from the reaction of various organic compounds at very high temperatures and pressures on the order of approximately 572.degree. F. (300.degree. C. and 2500 psi), (1,757,750 kg/m.sup.2). Maintenance of this temperature and pressure throughout the process stream is essential, since the polymer very quickly hardens into a virtually indestructible mass upon cooling. For this reason, polymer manufacturing facilities typically run twenty-four hours a day, seven days a week for several months. Therefore, a polymer manufacturing plant of relatively modest size will manufacture polymer in huge quantities. The many different uses for polymers require that they be manufactured according to many varying formulas. A typical polymer will contain several primary constituent parts and many secondary additives, often in minute quantities, which nevertheless have a significant affect on the qualities of the end product. For example, in polymers such as polyester and nylon, additives to the polymer mix reduce the coefficient of friction of yarn manufactured from the polymer so that yarn guides, rings and the like which come into contact with the rapidly moving yarn do not wear out rapidly. Other additives and formulation end groups in the polymer control the rate and extent to which the polymer absorbs and reacts with dyes. Still other additives affect the strength, elongation, moisture absorption rate and many other characteristics. Infrared analysis of polymer is typically carried out by forming a film or melt from polymer flake or pellets. The film or melt is allowed to cool and, when analyzed in the laboratory, is analyzed in its cool state. Infrared analysis of polymer at ambient temperature gives results which may differ considerably from analysis of the same polymer at its process stream temperature. This limits the utility of the information obtained. Even if the polymer melt is reheated, the results will still not provide a completely accurate reflection of the polymer melt in the process stream since, each time the polymer melt is heated, cooled and reheated, its chemical composition changes somewhat due to heat related reaction of the polymer components and the escape of volatile from the polymer caused by heating. Even if reasonably accurate results are achieved, the length of time which necessarily elapses between the taking of the sample, the completion of the infrared analysis in the laboratory and the correction of the formula can result in the manufacture of vast quantities of polymer which exceed quality control limitations and must be reprocessed, thrown away or sold as waste or second quality product.
Therefore, it is highly desirable to sample and carry out infrared radiation analysis of materials such as polymers on an on-line basis at the process stream.
It is necessary to maintain a polymer melt at its process stream temperature during FTIR analysis. Even momentary contact by the polymer stream with a relatively cooler object such as the probe will cause a relatively thick film or coating of polymer to form and cling to the probe. Thereafter, the infrared radiation is only sampling the stationary material clinging to the probe to a depth of 4 to 8 microns and not the material in the moving process stream.
The development of a sample cell which permits on-line chemical analysis of polymer melt in a moving process stream permits samples to be taken and quality variations detected with sufficient speed so that corrections can be made before significant amounts of waste or second quality polymers are produced. Furthermore, the development of such a sample cell permits the continuous sampling of the polymer melt. Such a continuous process permits the establishment of alarm limits which automatically alerts production personnel when the chemical composition of the polymer melt varies outside of specifications, diverts defective polymer melt out of the process stream for reprocessing or even, through suitable servo-mechanisms, controls upstream processes to bring the chemical analysis back within standards.
The present invention solves this problem by, in effect, taking a moving "slice" of a material, such as polymer melt and, while maintaining it at its precise process stream temperature and pressure, passing infrared radiation through it from one side to the other. The maintenance of the process stream temperature requires isolating the sample cell through which the material passes from the very sensitive equipment which characterizes the FTIR type of infrared analysis.
An infrared spectrum is a record of intensity of infrared radiation as a function of frequency or wavelength. A large number of variables affect infrared detection, including atmospheric absorption, a variation of source intensity with frequency, and changing dispersion in the spectrometer in the presence of contaminating substances in the environment. For this reason, the electrical output of an infrared detector is not constant even in situations where the sample is theoretically completely transparent.
To correct for these variations, it is necessary to determine two spectra--one with the sample in the radiation beam and one with the sample removed from the beam. The absorbtion as a function of frequency is then computed from these two spectra. In effect, the spectra with the sample removed from the beam is subtracted from the sample in the beam to leave a resulting spectroscopic "fingerprint" of the substance being examined. Typically, this is done by introducing a second optical path called a reference path which is ideally as nearly like the first beam as possible except for the absence of the sample. In laboratory analysis, this is a simple matter since the environment is controlled expressly because such analysis is taking place.
However, in analyzing materials such as molten polymer within a process stream, the sample must be placed within an oven which maintains the sample at the process stream temperature for analysis. Heretofore, the background or reference sample has been either not taken at all, or taken outside the environment of the sample cell. This produces variations and distortions into the process which result in a less than fully accurate infrared spectrum of the sample material.